Heretofore, in order to promote a complete combustion particularly at a low load in a small-sized direct injection type diesel engine, there has often been used a so-called re-entrant type combustion chamber in which, as shown in FIG. 1, a side wall 4a of a combustion chamber 4 which opens to a top 2a of a piston 2 is inclined divergently toward the interior of the piston 2.
More particularly, during a diffused combustion at the time of descent of the piston 2, it is very likely that flames ejected from the interior of the combustion chamber 4 to near the peripheral wall of a cylinder will contact an air layer Q (generally called a quench zone; hereinafter referred to also as "quench zone") of a relatively low temperature located near the peripheral wall of the cylinder and also contact a cylinder wall of a low temperature and will be cooled thereby, resulting in a complete combustion being not effected and carbon particles in the flames remaining unburnt, thus causing inconveniences such as an increased concentration of discharged smoke and a decreased output. To eliminate these inconveniences, the side wall 4a of the combustion chamber 4 is inclined as shown in FIG. 1 to prevent flames from entering the quench zone Q.
In such a re-entrant type combustion chamber, however, fuel spray from an injection nozzle 6 is easily concentrated on a lower portion of the combustion chamber 4 due to the inclination of the side wall 4a of the chamber 4 as shown in FIG. 1, thus causing a partial reduction of air utilization factor. As a result, the effect of reducing the concentration of discharged smoke is not attained to a satisfactory extent.
In such a re-entrant type combustion chamber, in order to disperse the fuel spray in an upper area within the combustion chamber 4, as shown in FIG. 2, it is necessary that the injection nozzle 6 is projected largely into the combustion chamber 4 to widen the injection angle .theta. to distribute the fuel spray after impingement against the wall surface also to an upper portion of the combustion chamber. However, as the amount of projection of the injection nozzle 6 increases, the temperature of the nozzle tip becomes more elevated, thus causing seizure of a valve seat face of the nozzle and a valve.
In the above re-entrant type combustion chamber, moreover, when the amount of fuel spray is large in a state of high load, the fuel spray is difficult to spurt out of the combustion chamber, thus causing shortage of air, reduction of output and increased concentration of discharged smoke.
In an effort to overcome the problem of air shortage at a high load, there has been proposed such a combustion chamber 8 as shown in FIG. 3 whose opening edge portion 10 is not throttled, namely, a so-called open type combustion chamber, whereby it is intended to let the fuel from the injection nozzle 6 flow out also to the cylinder portion outside the combustion chamber 8 in addition to the interior of the same chamber and to thereby utilize the air at the cylinder portion positively. In such an open type combustion chamber, however, combustion gas will strike on the quench zone Q from the combustion chamber 8 at a low load and thereby cooled to increase the amount of black smoke. For these reasons, in a small-sized direct injection type diesel engine, it has heretofore been difficult to obtain a combustion chamber capable of covering all conditions from high to low loads.